For large and small businesses power generators represent a considerable capital investment and support mission critical facilities and processes. Often their malfunction can lead to possible dangerous conditions and significant financial loss. Periodic generator maintenance and testing are vital to ensuring that each power station unfailingly performs as expected when called upon to do so.
USP&E's generator engineers know the importance of preventative maintenance practices and periodic operational testing of their genset units. Emergency gensets required by a building's safety code require minimum test conditions and frequency of testing are prescribed by applicable codes.
An important preventive measure of a comprehensive generator maintenance program is periodic load bank testing. Rarely is it required specifically by building coding but is often mandated for emergency generator units operating in critical areas like hospitals.
A load bank is a stationary or mobile piece of equipment designed to provide a simulation of the actual electrical load the power generator is supposed be capable of powering up to. Load bank testing shows the generator operator if the equipment is capable of handling the electrical demands that are required of the unit or if it needs an overhaul.
Power Factor Effect on Generators
Industrial electrical loads comprise resistance and inductive reactance, with the latter induced by all electro-magnetic devices. Inductive reactance causes current flow to lag the applied voltage by 90 electrical degrees. Solid state electronic devices are nonlinear and their current waveform does not conform to the applied voltage waveform. They, too, cause current to lag the applied voltage in the manner of electromagnetic loads.
The vector resultant of resistance and reactance is the circuit impedance. Current flow through the resistive component yields kilowatts (kW), and the kW level determines the horsepower load that will be advocated on the genset engine. The current flow through total impedance yields kilovolt-amperes (kVA), which converts directly into the heating that will be imposed on the power generator.
The circuit parameter known as "power factor," or PF, is the ratio of kW to kVA. Power factor is usually conveyed as a percentage integer value. A PF of 080, for example, is typically expressed as 80%. Power factor decreases with increased reactance, and increases as the ratio of resistance to reactance increases. In a purely resistive circuit, kVA is equal to kW, and PF is 1, or 100%.
There are quite few pure resistive loads in industrial generator power plants; their existence is essentially narrowed to incandescent lighting and resistance heaters. Polyphase induction motors have power factors ranging from 50% to 90%, with the lower power factors applying to smaller and lower speed generator motors.
The following recommendations for an overall engine/generator Preventative Maintenance program are general guidelines only, to be modified in accordance with specific site circumstances.Weekly
The system should be started and operated for 30 to 40 minutes, either with or without electrical load imposed on the unit. A visual check should be made for fuel and oil leaks, and any abnormal noises investigated. Engine fluids, gauges, and instruments should be checked.Monthly
The generator unit should be run for 30 to 40 minutes under actual load, and all weekly-check items observed. Engine cranking battery electrolyte levels should be checked, and battery specific gravity hydrometer readings should be thoroughly taken.Semiannually
Every 6 months or 250 engine-operating hours, a thorough inspection should be made of the entire cooling, fuel, starting, lubricating oil, and air intake/exhaust systems. Oil should especially undergo spectrophotometry and infrared analysis to confirm its integrity. System safety control systems and unit control panels should also be checked accordingly. The generator and exciter stator and rotor windings should be checked with a megohmmeter to detect any insulation degradation in the unit.Annually
Every year or 250 engine-operating hours, engine oil, and oil, air, and furl filters should be changed. An engine coolant condition analysis should also be performed accordingly. The engine/generator must go through a resistance/reactance load bank test over its full load range for a minimum of 2 hours.Biannually
Every 2 years, the engine coolant system should be thoroughly flushed and refilled accurately. Upper and lower radiator hoses, belts, and block heater hoses should be inspected and replaced.
Load Bank Test Verification of Voltage Regulator Operation
Only reactive load bank testing can verify the true performance of the generator or genset voltage regulator; the regulator is not fully challenged in a resistance-only load bank test alone. The voltage regulator is what allows the generator system to recover quickly from large electrical load changes. When large block loads are applied, engine speed drops for a moment before recovering to its steady-state condition.
This recovery interval is known as the "transient response". Recovery from transient response might not be possible if the regulators are not functioning properly. The generator magnetic field might even collapse, rendering the power generator useless in some cases.
Testing with a resistance/inductance load bank permits block loads to be introduced having the same encumbrance characteristics as actual in-service loads of a generator or genset.
Periodic Load Bank Testing Can Eliminate Many Unexpected Generator Problems
It is prudent to perform load bank testing on an annual basis, because it can eliminate most future problems caused by neglect and underutilization of the unit. Weekly test operation under no-load or light-load conditions does not permit engine fluids to attain proper operating temperature. Cooling system controls will not be fully exercised to proper levels, preventing coolant from circulating through radiators, and inviting failures stemming from generator inactivity. Engine safety shutdown systems are not subjected to actual operating conditions, and are therefore not fully tested.
Diesel-driven generator units are subject to a particular blight known as "wet stacking" or "slobbering" that stems from operation under no-load or light-load conditions. Such under operation causes diesel fuel deposits to collect on the combustion chamber, injector nozzles, piston rings, turbocharger, and exhaust system. The result is diminished generator engine output capacity. Regular load bank testing incinerates the accumulated deposits and preserves engine output capability.
USP&E Worldwide Load Bank Testing Services
Load bank testing is available worldwide through USP&E's service and maintenance network. We know some operations involve generators needing to be located in very remote regions of the world. USP&E has an international team of generator service experts that can perform your load bank service and a host of other generator services.